Instability of the HF Ground State in 56Ni Caused by -Like Four-Nucleon Correlations

The problem of whether the doubly-closed·shell state of 56Ni is stable or not against the 4p·4h excitations caused by the a-like four· nucleon correlations is investigated by testing the stability condition of the HF ground state. First, the stability condition of the HF ground state against the a·like four·nucleon correlations is reformulated. Second, the stability of the HF ground state in 56Ni is examined using the refined stability condition. The calculations with the use of available effective interactions indicate that the HF ground state with the double magic number Z = N = 28 is unstable against the a·like 4p-4h excitations.